Electrochemical cells utilizing monopolar or bipolar cell designs having reactive electrodes are well known. Conventional flow batteries are typically comprised of a "stack" of cells, an electrolyte pump, an electrolyte reservoir, a cooling element, and external studs in electrical communication with the terminal electrodes. Each cell is comprised of an electrode upon which the anodic reaction takes place and an electrode upon which the cathodic reaction takes place
In a monopolar battery, each electrode functions as a single pole, so that two separate electrodes (anode and cathode) are required to form an individual cell. The electrochemical potential between the anode and the cathode determines each cell voltage. An ion-permeable barrier, or separator, separates the anodic half cell from the cathodic half cell. The cells forming the stack are hydraulically isolated from each other and electrically connected in series by highly conductive straps which extend outside the battery body. A separate strap connects the anode of one cell to the cathode of an adjacent cell; there is no strap connecting an anode to the cathode of the same cell. The cells located at opposite ends of the stack each have an unconnected electrode, there being no adjacent cell. These unconnected electrodes function as the terminal anode and terminal cathode, respectively. Typically, a strap or stud connects each terminal electrode to a power supply (during charge) or to a load (during discharge).
Electron flow in a monopolar battery involves ionic and electrical transmission of electrons. Within each cell, electrons leave the cathode, travel ionically through the electrolyte and separator, and are deposited on the anode. The electrons then leave the body of the battery as they travel electronically through the strap to the cathode of the adjacent cell.
In a bipolar battery, on the other hand, each electrode comprises two "poles", such that the anodic reaction occurs on one side of the electrode and the cathodic reaction occurs on the opposite side of the same electrode. Thus, in contrast to a monopolar battery, which requires two separate electrodes per cell, a bipolar battery is comprised of bipolar electrodes upon which both the anodic and cathodic reactions occur. As with a monopolar battery, the cells in a bipolar battery are electronically connected in series. Unlike a monopolar battery, where the cells are hydraulically isolated, the cells of a bipolar battery are hydraulically connected in parallel.
Current flow in a bipolar battery also involves electrical and ionic transmission: (1) electrons travel ionically through the electrolyte and through the ionpermeable separator; and (2) electrons flow electrically between adjacent cells via a conductive substrate, i.e., the bipolar electrode, which is common to both the anodic and cathodic half cells.
Thus, whereas current typically "leaves" a monopolar battery as it travels through the straps, electron flow in a bipolar battery is entirely internal. As a result, bipolar batteries generally possess a higher current density than monopolar batteries because of decreased electrical resistance.
Zinc-bromine electrochemical systems have yielded varying degrees of success in bipolar flow battery applications. See Zito, U.S. Pat. No. 4,482,614, issued Nov. 13, 1984, the disclosure of which is hereby incorporated by reference. In a typical bipolar zinc-bromine battery, aqueous zinc bromide electrolyte is circulated throughout the various half cells. The zinc bromide solution contains a rich supply of positively charged zinc ions and negatively charged bromide ions. In addition, conductivity additives such as KCl and NH.sub.4 Cl are added to the solution to reduce ionic resistance and facilitate passage of the ions through the separators.
The battery stack is comprised of alternating electrodes and separators such that an anode half cell is disposed on one side of each electrode and a cathode half cell is disposed on the opposite side of each electrode. For each cell, one half cell contains the anolyte and the other half cell contains the catholyte. An anolyte pump urges the anolyte through a common anolyte manifold to each anodic half cell and a catholyte pump supplies the catholyte to each cathodic half cell through a parallel common catholyte manifold. The alternating separators and electrodes are sealed together in a manner which prevents communication between the anolyte and catholyte hydraulic systems.
In the discharged state, the electrolyte solution in the anode system is chemically identical to the electrolyte solution in the cathode system. During charging, the following reactions take place: EQU Zn.sup.++ +2e.fwdarw.Zn.degree. EQU 2Br.sup.- .fwdarw.Br.sub.2 +2e.sup.-
reactions proceed primarily to the right, with zinc plating at the anodic surface of each electrode in the stack. At the same time, molecular bromine is formed at the cathodic surface of each electrode. Upon being formed, the molecular bromine combines with a complexing agent to form a quaternary ammonium complex phase, or "second phase". The second phase is pumped through the system and stored in the catholyte reservoir. When the battery is fully charged, a supply of plated zinc is stored on one side of each electrode and a supply of complex bromine is stored in the catholyte reservoir.
Upon discharge, the following reactions occur: EQU Br.sub.2 +2e.sup.- .fwdarw.2Br.sup.- EQU Zn.degree..fwdarw.Zn.sup.++ +2e.sup.-
The discharge reactions proceed primarily to the right until all the plated zinc is removed from the electrode surfaces. During the discharge phase, the plated zinc is oxidized and the freed electrons pass through the electrode where they join with molecular bromine to form bromide ions. The positively charged zinc ions travel through the separator and remain in solution. At the same time, the bromide ions pass through the separator in the opposite direction and remain in solution. As the discharge reactions occur on opposite sides of each electrode, (1) complex bromine is reduced and (2) plated zinc is oxidized, resulting in a supply of electrons traveling ionically through the separators and electrolyte, and electrically through the electrodes.
At the end battery, the electrons contact the terminal , where the current is collected and transmitted to a load. The electrical and mechanical with which these electrons are transferred to the in large measure determines the practical utility of the battery. Thus, terminal electrode configurations are of considerable commercial importance.
Regardless of the particular configuration of the terminal electrode, some means must be provided whereby the load may physically attach to the battery. See, for example, Britz et al., U.S. Pat. No. 3,964,928, June 22, 1976 (Col. 5, lines 39-49). It is generally known to use a screen, mesh or honeycomb material as an electrode and to physically attach the current collector to the screen. See Otto, et al., U.S. Pat. No. 4,401,714, Aug. 30, 1983 (Col. 3, lines 40-48). A perforated plate having conductive metal elements disposed therein may be used as a current collector. See Rowlette, U.S. Pat. No. 4,658,499, Apr. 21, 1987 (Col. 4, lines 13-20). It is also generally known to place the terminal in electrical contact with the current collector. See FIGS. 1 and 2, and Column 3, lines 25-33 of Specht, U.S. Pat. No. 4,677,041, June 30, 1987.
Others have attempted to gather current from a terminal electrode and conduct it to the battery terminal by employing metal plates in surface to surface contact with the terminal electrode. See, for example, Bradley, U.S. Pat. No. 4,208,473, June 17, 1980 (Col. 6, lines 31-34; FIG. 5).
Presently known terminal electrode configurations, however, are unsatisfactory in several regards. For example, screens and other thin current collecting materials typically extend beyond the periphery of the electrode in the region where the terminal is attached to the collector. Repeated attachment of the load to the terminal tends to flex the thin collector material so that, over time, the material becomes weak or forms cracks, thus reducing the number of current paths and increasing electrical resistance. This is an especially persistent problem when screen is employed as the collector material, as screens are typically on the order of 0.003 inches thick.
In addition, in electrode configurations in which the current collector or any metallic portion of the terminal electrode is disposed external to the battery, corrosion problems arise.